The fabrication of electrodes for biological applications such as neural stimulation has gained increased attention in recent years. Biocompatible (e.g., implantable), flexible, thin-film, multiconductor microelectrode stimulation arrays have been investigated for use as, inter alia, complex neural prostheses, such as cochlear prostheses. See, e.g., White, et.al., Ann. N.Y. Acad. Sci , 405:183-190 (1983). This article describes some of the technological difficulties inherent in fabricating thin-film neurostimulating arrays. Such difficulties arise not because of an inability to control, by photolithographic technology, the dimensional resolution of the arrays, but rather because of the inability of the art to fabricate the stable, flexible and durable composite (e.g., metal-polymer) materials necessary for preparing such arrays.
For example, multiple conductor miniature electrodes that are used for stimulating the residual nerve fibers of an impaired human cochlea or inner ear are subjected to demanding clinical conditions. They require long-term reliability and stability of the metal-polymer interface, as well as the ability to provide a suitable charge transfer from stimulating electrodes having geometrically small surface areas.
It is desirable that such electrodes would ideally be made up of both a noble metal conductor, such as platinum, and a polymeric support, such as polyimide. Both of these materials are presumed to be biocompatible, and are known to be bioinert. This very inertness however makes it extremely difficult to attach the two materials directly to each other in a manner that will enable the resulting composite to undergo conventional photolithography, as well as to then withstand the rigors of a biological environment. The authors of the above White et.al. article recognized this problem, and attempted to solve it by the use of a thin intermediate layer of tantalum or titanium between the surface of the polymer substrate and the platinum layer. Delamination of platinum from the polyimide substrate continued to be a vexing problem however, see e.g., White, "System Design of a Cochlear Implant", IEEE Engineering in Medicine and Biology Magazine, Vol. 6, No. 2, pp. 42-46 (1987).
Other authors have similarly expressed the frustration and difficulty of this problem. For instance, Roberts et.al., 2nd Quarterly Progress Report, Jan. 1, 1984 through Mar. 31, 1984, NIH Contract NOl-NS-3-2352, states that "[m]etal to polyimide adhesion after electrical stimulation continues to be a difficult and elusive property to achieve." This report comments on how a composite can appear to exhibit good adhesion under a saline soak test, but fail as soon as electrical stimulation is applied. Since such conditions are used to simulate those encountered in biological applications, this report illustrates the need to fabricate flexible thin-film composite articles that are suitable for preparing electrodes for use in such biological environments and other demanding applications.
Researchers at the solid state Electronics Laboratory of the Bioelectrical Sciences Laboratory, University of Michigan, have explored the development of polyimide-tantalum thin film conductor cables. Adhesion of metals to polyimide, and saline durability, are described as major problems. "Multichannel Multiplexed Intracortical Recording Arrays", Quarterly Report #1, (Contract NIH-NINCDS-NOl-NS-7-2397) (February 1988).